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The characterization of the strength and fracture toughness of three common structural fibres, E-glass, AS4 carbon and Kevlar KM2, is presented in this work. The notched specimens were prepared by means of selective carving of individual fibres by means of the focused ion beam. A straight-fronted edge notch was introduced in a plane perpendicular to the fibre axis, with the relative notch depth being a 0 / D ≈0.1 and the notch radius at the tip approximately 50 nm. The selection of the appropriate beam current during milling operations was performed to avoid to as much as possible any microstructural changes owing to ion impingement. Both notched and un-notched fibres were submitted to uniaxial tensile tests up to failure. The strength of the un-notched fibres was characterized in terms of the Weibull statistics, whereas the residual strength of the notched fibres was used to determine their apparent toughness. To this end, the stress intensity factor of a fronted edge crack was computed by means of the finite-element method for different crack lengths. The experimental results agreed with those reported in the literature for polyacrylonitrile-based carbon fibres obtained by using similar techniques. After mechanical testing, the fracture surface of the fibres was analysed to ascertain the failure mechanisms. It was found that AS4 carbon and E-glass fibres presented the lower toughness with fracture surfaces perpendicular to the fibre axis, emanating from the notch tip. The fractured region of Kevlar KM2 fibres extended along the fibre and showed large permanent deformation, which explains their higher degree of toughness when compared with carbon and glass fibres. This article is part of the themed issue ‘Multiscale modelling of the structural integrity of composite materials’.

Self-healing materials are a very promising kind of materials due to their capacity to repair themselves. Among others, dichalcogenide-based materials are widely studied due to their dynamic covalent bond nature. Recently, the reaction mechanism occurring in these materials was characterized both theoretically and experimentally. In this vein, a theoretical protocol was established in order to predict further improvements. Among these improvements, the use of diselenides instead of disulfides appears to be one of the paths to enhance these properties. Nevertheless, the physicochemical aspects of these improvements are not completely clear. In this work, the self-healing properties of several disulfides, diselenides, and mixed S–Se materials have been considered by means of computational simulations. Among all the tested species, diphenyl diselenide based materials appear to be the most promising ones due to the decrease on the reaction barriers, instead of weaker diselenide bonds, as thought up to now. Moreover, the radical formation needed in this process would also be enhanced by the fact that these species are able to absorb visible light. In this manner, at room conditions, selenyl radicals would be formed by both thermal dissociation and photodissociation. This fact, together with the lower energetic barriers needed for the diselenide exchange, makes diphenyl diselenides ideal for self-healing materials.

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